Due to hoisting, the value of a 'var'-declared variable may actually change even
if the code contains only the "initial" assignment, namely when that assignment
occurs inside a loop.  For example:

  let i = 10;
  do { var x = i } while (i--):

As a simple and very conservative approximation of this, we explicitly mark
as maybe-assigned any non-lexical variable whose "declaration" does not
syntactically occur in the function scope.  (In the example above, it
occurs in a block scope.)

BUG=v8:5636

Review-Url: https://codereview.chromium.org/2673403003
Cr-Commit-Position: refs/heads/master@{#42989}